This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
182. An architect should, if possible, examine the brick to be used in a building before they are laid in the wall, and they should meet the following requirements:
1. They should be sound, free from cracks or flaws, and from stones and lumps of any kind, especially pieces of lime.
2. The bricks must be uniform in size, with sharp angles and edges, and the surfaces true and square to each other; this insures neat work.
3. Good building brick should be quite hard and well burned. A simple, and generally satisfactory test for common brick is to strike two of them together, or to strike one with the edge of a mason's trowel; if the brick gives a ringing sound it is generally sufficiently strong for any ordinary work. A dull sound shows the brick is soft or shaky.
4. The quantity of water absorbed is also important. A good brick should not absorb more than one-tenth its weight in water. A good test is to weigh the brick first, then immerse it in water for twenty-four hours, and weigh it again. From the increase in weight the percentage of water it has absorbed may be found. Very soft underburned brick often absorb from 25 to 35 per cent. of water. Weak, light-red brick, often used in filling the interior of walls, will absorb about 20 to 25 per cent., while the very best brick may absorb not more than 5 per cent., and should, if possible, be used for outside walls and foundation walls and piers.
5. Brick that are suitable for piers and the foundations of heavy buildings should not break under a crushing load of less than 4,000 pounds per square inch.
6. The transverse strength of a brick is quite as important as the crushing strength. A good brick, 8 inches long, 4 inches wide, and 2 1/2 inches thick, should not break under a center load of less than 1,600 pounds, the brick lying flat, supported at each end only, and having a clear span of 6 inches, and a bearing at each end of 1 inch. A first-class brick will carry 2,250 pounds in the center and not break. Tests have been made with brick that carried 9,700 pounds before breaking.